Transcription Unit - Molecular Basis of Inheritance, Biology, Class 12 Video Lecture

hello friends this video on molecular
basis of inheritance part 19 is brought
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from exam so now we will talk about
transcription unit now where exactly in
a DNA the process of transcription takes
place as I mentioned before also that it
is not that the entire DNA gets replaced
with an RNA it is not like that it is
just a portion of DNA which where one
strand of DNA will act as a template and
it will generate a portion of RNA so let
us and that unit contour that part of
DNA is known as the transcription unit
so it is basically that portion of DNA
which transcribes and give rise to RNA
so the stretch of DNA which gets
transcribed to RNA molecule is term as
transcription unit so the process of
transcription takes place only within
this portion of the DNA now there are
three important regions which define a
transcription unit so when you say a
transcription unit so it is being
divided into three major important
regions so let us see what are they
promoter there is a promoter up side
suppose if this is the DNA so we are
just trying to show you that that
portion which is called transcription
unit so this is the promoter now
basically you can see being head
structure it is not substructure it is
just this much portion of the DNA that
is what I was trying to say next is the
structural gene so here you can see this
portion of the DNA represents the
structural gene and the last one is the
terminated so this much portion of the
DNA will act as the Terminator and this
portion is the promoter now as the name
suggests promoted will denote the
beginning of transcription and that
transcriptional gene represents all
those genes which will get transcribed
so in this portion of the
transcriptional gene you will have you
will see that one of the strands of DNA
will behave as the M
and other one would be the holding
strength and finally there is the
terminated region where the process of
transcription will stop so this entire
unit has known as the transcription unit
so let us try to understand the function
and the structure of each of the units
on each of the regions in the
transcription unit so you start with
promoter so it is the region of DNA that
initiates the transcription of a gene so
this portion now before this portion it
was not a part of transcription unit so
there you know there is no transcription
that will happen here only after this
region transcription starts so it
initiates the process of transcription
so now let us make use of a simpler
diagram to explain the different regions
of the transcription unit now instead of
having the double helix structure of DNA
decision that these are the two strands
of DNA which wrap around each other to
form the double helix structure so this
black one is the template strand and the
pink one is the coding strand so let us
suppose that the template strand is
running from 3 prime 5 prime direction
and the coding strand is just the
Barbossa that escribe prime to 3 prime
direction so if you talk about promoter
this is where the promoter lines so this
one is the promoter so it provides
binding site for RNA polymerase like
howard DNA replication the leading role
was played by an enzyme called DNA
polymerase similarly in the process of
transcription the leading role is played
by RNA polymerase so RNA polymerase will
bind to the transcription unit at the
promoter so promoter will provide that
whole area where any polymer is can bind
only ones RNA polymerase brain kind it
can start the process of transcription
it is located upstream on DNA so
you see upstream that is towards the
other side that isn't like how if you
talk about as a squirrel in the river so
we say it is flowing up uphill so from
the top of the hill to the bottom of the
hill so if you have some water body so
towards the upper side you call it
uphill or upstream where it starts and
towards the lower side is the downstream
so similarly here where the entire
transcription unit starts that is the
upstream and where the transcription
unit ends that is towards the Terminator
side that is the downstream of DNA so
promoter is located upstream on DNA
it is located towards the five prime end
of structural gene now how will you
define which is the five prime end
because the coding strand has and the
template strand their notations are
opposite to each other because they are
anti-parallel strand now whenever we
denote the which end of the entire
transcription unit we take the coding
strand as the basis so whichever is the
five prime end of coding strand that is
considered as the five prime end of the
transcription unit so this is just for
convenience because otherwise you create
confusion
so promoter is always located towards
the five prime end of the coding strand
so wherever five prime of coding strand
promoter will be backside so obviously
tell me true nature will be on the other
side so its length can be up to 100 mm
base players now this promoter is
nothing but a region it is not some
other additional substance it is just a
region or D on DNA so here also there
will be many base clear so the number of
base pairs in the promoter region can be
up to 100 mm base pair so it can extend
up to that much of stretch
next is the structural gene these are
the gene that codes for any adding or
structural protein so all the genes
which are located in between this
portion they are structural genes and
they are going to add for our innate so
the base sequence in these genes are
going to dinner they are define the base
sequence
I am in strength and the third word is
the Terminator so here you have the
Terminator so this is terminator and
it's this region defines the end of the
transcription
so basically when the a DNA polymerase
comes it by each two promoter it passes
on through the structural gene or while
he passes on through the structural gene
it creates a copy of RNA and when it
reaches the terminator it gets a signal
that this is the end of transcription so
the process of transcription stops it is
located downstream on the DNA so this is
the upper side this is the down side it
is located towards three prime end of
the structural gene as I said five prime
end promoter three prime end is
terminator so please remember that all
the location reference is always taken
with respect to the coding strand so
please do not get confused regarding
that
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